An enzyme, found in activated macrophages and certain other tissues, that catalyses the production of the highly reactive molecule nitric oxide (NO). Macrophages that have ingested bacteria or other pathogens are prompted by helper T cells (T_H1 cells) to destroy those pathogens. γ‎-Interferon (IFN-γ‎) secreted by the T_H1 cell, in combination with tumour necrosis factor α‎ (TNF-α‎) from the activated macrophage, causes increased expression of the iNOS gene, and hence synthesis of the enzyme, leading to production of NO. This has broad antimicrobial activity: it can be both used inside phagocytic vesicles within the macrophage and released outside the cell, for example to attack large pathogens, such as parasitic worms. Expression of iNOS also occurs more widely during inflammation and wound healing.